Vinyl chloride polymerizates prepared by emulsion polymerization are mainly used for the preparation of plastisols, i.e. dispersions of the polymerizate in plasticizers. These plastisols or pastes are widely used in moulding processes, e.g. slush moulding, rotational casting, in coating processes, e.g. dipping, spread coating, or for the production of cellular products. The desired products are obtained by gelation of the plastisol, which can also contain stabilizers, fillers, pigments, etc., through heating.
The stability of these plastisols is to a great extent dependent on the average particle size and the particle size distribution of the polymerizate. It is essential that the plastisol have a low initial viscosity and only a slow
In the emulsion polymerization the monomer or the mixture of monomers is dispersed in water to a droplet size within the range of 10 to 30 .mu.m by means of an emulsifier and the polymerization is carried out with water soluble initiators. The initiator decomposes at the polymerization temperature forming water-soluble radicals. These react with a monomer molecule dissolved in the aqueous phase and radicals of oligomers are formed. At a certain chain length the growing polymer chain is precipitated and particles, which are stabilized by a water-soluble emulsifier, are formed. After this initiation phase the polymerization proceeds in the monomer swelled particles. If the emulsifier content of the aqueous phase is above the critical micelle concentration the initiation can on the other hand occur in monomer-containing micelles. Hereby are formed polymer particles in which the polymerization then proceeds. The obtained polymerizate is in the form of latex and is isolated by breaking the latex or by evaporation in a spray drier or on a roll drier.
Vinyl chloride polymerizates prepared according to this process generally have a relatively narrow particle size distribution and the primary particles have a very low diameter with the result that plastisols produced from these polymerizates show a very high viscosity and their application fields are thus very much restricted. In order to obtain plastisols having suitable viscosity properties and ageing properties it is often necessary to mix polymerizates of different particle sizes. Another method which has been technically applied is to carry out the emulsion polymerization in the presence of a seed latex whereby a broader particle size range, and thereby a lower viscosity of the paste is obtained.